The invention relates generally to semiconductor devices, and more particularly to semiconductor capacitor constructions and methods of forming semiconductor capacitors, particularly in applications for forming dynamic random access memory (DRAM) cell structures and integrated circuitry incorporating DRAM cell structures.
The continuing densification and miniaturization of integrated circuits has led to smaller areas that are available for semiconductor memory devices. For example, in the fabrication of high density dynamic random access memory cells (DRAMs), there is less area available for the storage node (capacitor) of a memory cell. However, the capacitor must have a minimum storage capacitance to ensure operation of the memory cell. There is also a need for increased storage to enable devices to perform more functions at a faster rate.
Several techniques have been developed to increase the storage area of the capacitor within a limited space. For example, surface area has been increased by forming the capacitor in a trench or as a stacked structure. The surface area of the capacitor has also been achieved by increasing the surface roughness of the lower electrode that forms the storage node.
One prior art process for increasing the electrode surface area by forming a rough upper surface is illustrated in FIGS. 1A-1D, with respect to forming the lower electrode as a layer of hemispherical grain (HSG) polysilicon. Referring to FIG. 1A, a semiconductor wafer fragment 10 is shown in a preliminary processing step to form a DRAM capacitor. Wafer fragment 10 comprises a semiconductor material 12 (e.g., monocrystalline silicon) and wordlines 14, 16, having nitride spacers 18 formed laterally adjacent thereto. A diffusion region 20 within the substrate material 12 is positioned between wordlines 14, 16, and electrically connected by the transistor gates that are comprised by wordlines 14, 16. An insulative layer 22 such as borophosphosilicate glass (BPSG) has been formed over the semiconductive material 12 and the wordlines 14, 16. A doped polycrystalline plug 24 has been formed through the insulative layer 22 to provide electrical contact between the capacitor and a diffusion region 20 between wordlines 14, 16. A contact opening 26 has been formed through the insulative layer 22 to the plug 24. A thin, heavily doped and substantially amorphous or pseudo-crystalline silicon layer 28 has been deposited over the insulative layer 22 and plug 24.
Referring to FIG. 1B, according to the prior art process, an undoped amorphous or pseudo-crystalline silicon layer 30 is deposited over the doped amorphous or pseudo-crystalline silicon layer 28. The wafer fragment 10 is then exposed to a silicon source gas such as silane or disilane (arrows 32) to form a seed layer of silicon crystals or nucleation centers that are introduced into and distributed over the surface of the undoped amorphous or pseudo-crystalline silicon layer 30, as shown in FIG. 1C, to facilitate subsequent hemispherical grain growth. The wafer fragment 10 is then thermally annealed to convert the undoped amorphous or pseudo-crystalline silicon layer 30 into crystalline structures that are facilitated by the randomly distributed silicon crystals of the seed layer. The thermal treatment causes the polycrystalline silicon to agglomerate around the seed crystals and form HSG polysilicon 34, resulting in the storage node structure 36 shown in FIG. 1D. Although not shown, the DRAM cell is then completed by forming a thin cell dielectric layer over the structure, followed by the formation of a second cell plate (i.e., top electrode), typically a conductively doped polysilicon or metal-based layer.
Although the HSG polysilicon increases the surface area of the lower capacitor electrode, current HSG-type methods for increasing capacitor surface area are approaching physical limitations. A disadvantage of using HSG silicon to form a container type capacitor structure) is that morphology needed to increase surface area is a function of inexact physical conversion of conductive films. HSG silicon morphology required to gain surface area enhancements needed for next generation part types borders on over-consumed, bulbous grain formations that are structurally unsound. Current technology does not allow ordered HSG silicon formation, and unwanted patterns from temperature gradients across the wafer and from gas flow dynamics create large variability in surface area enhancement. Inexact ordering and size of converted grains can be problematic. For example, the grains of the silicon overgrow and form discontinuous and isolated islands. Further, if HSG silicon growth is too extensive and extends to the opposing sides of the container, the surface area of the capacitor plate decreases. In addition, since seeding is not instantaneous and takes a finite and prolonged amount of time, grains formed at the beginning of seeding are larger than grains formed from seeds deposited at the end of the seeding step. It would be desirable to have more precise and uniform roughness provided over the surface of the capacitor plate to increase surface area.
The present invention relates generally to semiconductor fabrication techniques and, more particularly, to the formation of a capacitor electrode.
In one aspect, the invention provides methods of forming a lower electrode structure in a capacitor of a semiconductor device. In one embodiment of the method, a texturizing layer in the form of a nanorelief or nanoporous film is formed prior to deposition of the cell conductive layer to form the lower electrode. The texturizing layer can comprise an ordered array of nanostructures and/or periodic network of surface structures having substantially uniform dimensions (e.g., height, size).
In another embodiment of the method, a polymeric material is deposited over the insulative layer of a container as a precursor that is converted to relief or porous structures upon ozonolysis and UV exposure, resulting in a textured layer comprising an insulative silicon oxycarbide film. The polymeric material comprises a hydrocarbon block and a silicon-containing block. The volume fraction of the hydrocarbon block relative to the silicon-containing block can be varied to form the nanostructures as a relief structure or a porous structure. The film is punch-etched (e.g., RIE) to clear an opening to the underlying substrate or conductive plug at the bottom of the cell for the subsequent deposition of a conductive material (e.g., polysilicon, conductive metal), resulting in a lower electrode have an upper roughened surface. After formation of the lower capacitor electrode, the structure is further processed to complete the capacitor by depositing a dielectric layer and forming an upper capacitor electrode over the dielectric layer. The capacitor can usefully be integrated into a DRAM cell.
In another embodiment of a method of the invention, a texturizing underlayer is fabricated from a conductive material prior to depositing a conductive layer to form the lower electrode. In forming the texturizing underlayer, a first conductive metal is deposited over the insulative layer of a container, a second dissimilar conductive metal is deposited over the first metal layer, and the two metal layers are annealed resulting in a textured layer comprising surface dislocations in a strain relief pattern, which is preferably a periodic and ordered array of nanostructures. A conductive metal is then deposited in gas phase over the texturizing layer whereby the depositing metal agglomerates onto the surface dislocations forming island clusters. Preferably, the surface dislocations of the texturizing layer are formed as a periodic network, and the overlying conductive layer comprises ordered arrays of metal island clusters. The capacitor can then be completed by depositing a dielectric layer and forming an upper capacitor electrode over the dielectric layer. The capacitor can likewise be integrated into a DRAM cell.
In another aspect, the invention provides a capacitor. The capacitors can be incorporated into a semiconductor circuit, including a circuit comprising a DRAM cell. In one embodiment, the capacitor comprises a lower capacitor plate comprising a conductive layer (e.g., polysilicon, conductive metal) overlying a texturizing layer that comprises, for example, silicon oxycarbide ceramic nanostructures, a dielectric layer overlying the lower capacitor plate, and an upper capacitor plate overlying the dielectric layer. The nanostructures of the texturizing layer can be formed by ultraviolet irradiation and ozonolysis of a polymeric material comprising a hydrocarbon block and a silicon-containing block. The nanostructures can be provided as pores or relief structures such as struts by varying, for example, the volume fraction of the hydrocarbon block relative to the silicon-containing block of the polymeric material.
In another embodiment, the capacitor comprises a lower capacitor electrode comprising a conductive metal layer overlying a conductive metal texturizing layer, a dielectric layer overlying the lower (bottom) electrode, and an upper electrode (e.g., polysilicon, conductive metal) overlying the dielectric layer. The texturizing layer comprises surface dislocations comprising a conductive metal that are formed by annealing overlying layers of a first and second conductive metal. A conductive metal layer is formed from a gaseous deposit of a conductive metal that agglomerates into clusters on the surface dislocations of the texturizing layer. Preferably, the texturizing layer comprises a periodic network of the surface dislocations, and the overlying conductive layer comprises ordered arrays of metal island structures.
In another aspect, the invention provides an integrated circuit (IC) device that incorporates either of the foregoing capacitors. The IC device can comprise, for example, an array of memory cells, internal circuitry, and at least one capacitor formed in a container and in electrical contact with an active area within a semiconductive substrate of the memory cell array. In one embodiment, the capacitor comprises a lower capacitor plate comprising a conductive layer overlying a texturizing layer comprising nanostructures composed of a polymeric silicon-comprising ceramic formed by UV irradiation and ozonolysis of a polymeric material comprising a hydrocarbon block and a silicon-containing block. In another embodiment of an IC device, the lower capacitor electrode of the capacitor can comprise a texturizing underlayer comprising surface dislocations formed from an annealed layer of two overlying and dissimilar conductive metal layers, and an overlying conductive layer comprising agglomerated island clusters of a conductive metal formed from the gaseous deposit of the conductive metal onto the surface dislocations of the texturizing layer.
Advantageously, the methods of the present invention eliminate the need for random seeding and thermal process conversion as required with respect to an HSG seed layer formation, and provides a greater level of control over topography ordering of the underlayer of a lower capacitor electrode. Unlike prior art methods of forming a capacitor using HSG silicon formation, the texture of the capacitor resulting from the present process is not a randomly created feature, and has a generally uniform pattern and height. Consequently, the size of the capacitor can be more definitively designed and engineered, which facilitates the fabrication of the memory cell. In addition, with the present methods, morphology needed to increase surface area is not a function of the physical conversion of a conductive film as in the case of HSG silicon films, and allows simple deposition of conformal conductive layers over a pre-existing topography. In addition, the temperatures required for HSG silicon deposition and thermal conversion are higher than any back end-of-line materials (metals) can tolerate, which limits formation of the capacitor to before such low temperature materials are deposited. The present processes can create ordered roughness at low temperatures and therefore have the advantage of allowing the capacitor to be formed anywhere in the process flow.